This invention relates to apparatus and methods for providing highly stable deep cryogenic temperatures and for enabling rapid thermal cycling at cryogenic temperatures.
Suitable apparatus for providing deep cryogenic temperatures include cryogenic refrigerators, also referred to as cryocoolers. To attain temperatures near absolute zero degrees Kelvin, a known available working cooling fluid is helium (He). The term “cooling fluid” as used herein refers to the working coolant (e.g., He) whether in a liquid, gaseous or any intermediate state and “degrees Kelvin” may be denoted herein by the capital letter “K”.
A variety of different thermodynamic approaches are used in commercial helium-cycle cryocoolers, including Gifford-McMahon (GM), pulse tube, and Stirling cycles. See, for example, “Cryocoolers: The State of the Art and Recent Developments”, R. Radebaugh, J. Physics Condensed Matter, vol. 21, 164219 (2009).
Known cryocoolers of the type shown in Prior art FIG. 1A suffer from temperature oscillators/variations (about a set temperature) as shown in FIG. 1B. The temperature variations present in the system of FIG. 1A may be substantially reduced by the introduction of a thermal damper as shown in FIG. 2. Adding the thermal damper of FIG. 2 to cryocoolers of the type shown in FIG. 1A, functions to add thermal capacitance to the system which reduces the thermal oscillations shown in FIG. 1B. However, the thermal damper functions to slow down the cooling response of the system which is undesirable in applications where it is desirable and/or necessary to have rapid cycling between two different (cryogenic) temperature levels. The need for a faster dynamic response conflicts with the desirable and/or necessary condition that once the operating temperature level is set, it be and remain very stable (i.e., that it not vary significantly or substantially with time).
The problems discussed above may be better understood with reference to FIGS. 1A, 1B and 2. FIG. 1A shows an insulating vacuum enclosure 230 containing a first, intermediate temperature, cooling stage 240 and a second, low temperature, cooling stage 260. FIG. 1A also shows, in a highly simplified form, apparatus (compressor 150, high and low pressure lines 153 and 155, cryocooler ambient stage 160 and pistons 242 and 262) for distributing cooling fluid (e.g., helium gas) to operate the first and second cooling stages (240, 260) and produce the desired cryogenic temperatures. When operational, cooling stage 240 may function to produce an intermediate temperature in the range of 40-70 K and cooling stage 260 may function to produce temperatures in the range of less than 3K to more than 10 K. The use of two stages is merely illustrative; some cryocoolers may have only one stage, while others may have three or more cascaded stages.
A device to be cryocooled, DUT 226, which may, for example, be a superconductive integrated circuit, (SIC), is thermally linked to the cold stage 260; (container 220 in FIG. 2). A thermometer or temperature sensor, 224, and a resistive electrical heater 228 are shown attached to stage 260; (container 220 in FIG. 2).
A problem with the cryocooler system of FIG. 1A is that it is operated with a low frequency cyclic process (e.g., on the order of 1 Hz) which in turn causes the cooling power to oscillate/vary at this frequency. Depending on the heat generation and the thermal mass (heat capacity) of the system, the temperature of the coldest stage (e.g., 260 in FIG. 1A) oscillates or varies. For example, a two-stage GM cryocooler of the type shown in FIG. 1A typically exhibits peak-to-peak temperature oscillations/variations of the order of 0.3 K (actually, 0.25K in FIG. 1B), or more, at temperatures of about 4 K, (e.g., 3.35K to 3.6K shown in FIG. 1B). In many applications, such as for cooling superconducting devices (e.g., device 226 in FIGS. 1A and 2) which have a strong temperature dependence, these temperature oscillations (e.g., of about 0.3K) are problematic since they may cause malfunctions of the devices.
Thus, it is desirable and/or necessary to have a very steady (substantially non-varying) operating cryogenic temperature (e.g., 4K) for proper operation of certain devices (e.g., superconductive circuits, superconducting magnets). A desired operating temperature (i.e., Td) may be selected or set within a predetermined range; but, once Td is reached, it is desirable to maintain the temperature within narrow margins (typically of order 1% or less). Variations/oscillations about the value of Td, even if relatively small, are undesirable because the operation of the devices (e.g., superconductive devices) being cooled is temperature dependent and is adversely affected by temperature variations.
It is therefore desirable and/or necessary to reduce thermal oscillations or variations of the coldest stage.
FIG. 2 shows a “thermal damper” apparatus which can be used to thermally dampen the temperature oscillations exhibited with the coldest stage of the system of FIG. 1A. The thermal damper includes a cryogenic fluid container 220 connected to a room-temperature helium gas reservoir 200 via a narrow capillary tube 210 to enable cooling fluid (helium gas) to flow between the reservoir and the container as a function of their respective pressures. Container 220 is thermally linked to the cold stage 260 via a thermal linkage 270. Gas reservoir 200 is located external to enclosure 230 and is operated at room temperature. Reservoir 200 provides a volume of gas which can flow in and out of cold container 220 and enables the sizing and construction of container 220 to be simpler and more practical.
As is known, He is a high thermal capacitance material at low temperatures. FIG. 2 shows that the gas in the thermal damper is physically separate from the working fluid of the rest of the cryocooler. The pressure of a fixed volume of He increases by more than a factor of 100 between 4K and 300K (room temperature), so that it is impractical to seal a sufficient quantity of He into a small volume in the cryogenic assembly while it is warm; the pressure would be much too large and would present a serious safety hazard. Instead, the cold container 220 is connected via a narrow capillary tube 210 to a larger gas reservoir 200 kept at room temperature.
For efficient operation, the capillary tube 210 is shown to be thermally linked to an intermediate cold stage 240 via a thermal linkage 250. The capillary tube may be formed of a low-thermal conductivity material such as stainless steel, so that in normal operation the tube itself does not transfer significant heat from room temperature to the cold stages.
Starting with the FIG. 2 system at room temperature, cooldown from room temperature is initiated by applying electrical power to the cryocooler (this includes powering ambient stage assembly 160 and compressor 150). Cooling stages 240 and 260 begin to cool down. This in turn causes the volume and the pressure in cold container 220 to decrease, causing additional gas from the room-temperature gas reservoir 200 to pass through the capillary tube 210 to the cold container 220. The cooldown process from room temperature continues until the cooling stage 260 reaches a desired temperature (e.g., Td is equal to 4K). Once Td is reached, the thermal capacitance of container 220 functions to reduce the amplitude of temperature oscillations/variations to very low levels (e.g., about 20 milliKelvins peak-to-peak) which is acceptable for operation of the device 226 being cooled.
However, in at least one respect, the system of FIG. 2 has a significant shortcoming. As is known, superconducting integrated circuits (SICs) based on rapid-single-flux-quantum logic (RSFQ) are very sensitive to the trapping of magnetic flux due to current transients and stray magnetic fields, which may prevent the proper operation of the SICs upon cool down. One solution to this problem is to thermally cycle the superconducting integrated circuits (SICs), from a desired operational temperature (e.g., a Td of 4K) to a “defluxing” temperature (TO greater than 10 K [at which temperature the niobium (Nb) superconductor reverts to its resistive state] permitting the trapped flux to escape. The system is then re-cooled down to 4 K to determine if proper operation has been attained. The process of raising and lowering the temperature (thermal cycling) is referred to as a thermal “deflux” cycle. If a first raising and lowering of the temperature is not successful, this “deflux” or defluxing thermal cycling may be repeated multiple times (as many as 10 or more times) until proper operation of the superconducting IC is achieved. However, the cooling cycle from 11K to 4K can be quite time consuming due in part to the use of the thermal damper. Each deflux cycle may take 30 minutes or more.
Thus, while thermal damping helps to maintain the desired operating temperature (e.g., Td) fixed (i.e., with very low levels of temperature oscillations), there are applications where thermal damping impedes with the need to rapidly cycle the temperature between a first temperature (e.g., the operating temperature Td) and another temperature (e.g., a higher temperature, Tf) to reduce, eliminate or minimize certain problems (e.g., trapped flux).
Therefore, a need exists for apparatus which can dampen temperature oscillations of a system, while allowing a rapid response of the system when the system is subjected to temperature cycling between different temperature levels. This is of particular importance where, for optimum operation, the temperature of certain devices being cooled must be operated at different temperature levels.